MTF - 40mm F2.5G
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MTF - 40mm F2.5G

Hypothetical MTF plots. F2.5 (Left) and F8 (Right)

The 40mm F2.5G also exhibits outstanding contrast (orange) across the frame wide open (left graph), which improves to essentially perfect performance upon stopping down (right graph). Sharpness (green) is also very high centrally and for much of the frame - though not as high as the 24mm prime - hovering around 85 - 90% for much of the imaging circle. Like the 24mm F2.8G, sharpness drops as you reach the peripheries of the image, and improve more significantly upon stopping down to F8. In fact, by F8, contrast and sharpness across the frame is outstanding, essentially at 90% or higher for both figures across the entire frame.

The 40mm prime requires less digital distortion correction than the 24mm, albeit significantly more than the 50mm, so we're not surprised to see more of an increase in edge sharpness upon stopping down.

A note on reading these graphs: sagittal or radial traces are solid while tangential traces are dotted. Orange traces are for 10 lp/mm detail, often indicative of lens' contrast performance. Green traces are for 30 lp/mm detail, often indicative of lens' sharpness performance. The horizontal axis ranging from 0 - 24 at the bottom of each graph represents radial distance (mm) from the optical center of the lens, while the vertical axis represents % contrast. The higher the traces, the better.